Polycarbonate resins formed of 2,2-bis(4-hydroxyphenyl)propane (popular name: bisphenol A) have been used in various optical material applications such as substrates of CD or DVD, optical films, optical sheets, a wide variety of lenses, or prisms since they are excellent in transparency, heat resistance, low water-absorption properties, chemical resistance, mechanical characteristics, and dimension stability. However, the resins formed of bisphenol A have large birefringence, and it is difficult to use them in the technical fields requiring low-birefringence.
Therefore, in the technical fields requiring low-birefringence, acrylic resins, noncrystalline polyolefins, or polycarbonate resins having a specific structure have been used. However, acrylic resins suffer from high water-absorption properties, low dimension stability, or low chemical resistance properties; and noncrystalline polyolefins suffer from low impact resistance, low chemical resistance properties or high price. Furthermore, some of molded products formed of acrylic resins or noncrystalline polyolefins don't exhibit sufficient low-birefringence, and therefore, in the technical field requiring lower-birefringence, such resins may not be used.
Examples of the polycarbonate resin having a specific structure include copolymerization-polycarbonate resins derived from 9,9-bis(3-methyl-4-hydroxyphenyl)fluorene and tricyclo[5.2.1.02.6]decanedimethanol (see Patent Document 1). Although injection-molded products formed of the resins are excellent in low-birefringence, they suffer from low impact resistance strength.
Examples of the polycarbonate resin having a specific structure include also copolymerization-polycarbonate resins derived from 9,9-bis(4-(2-hydroxyethoxy)phenyl)fluorene and a prescribed hydroxy compound (see Patent Document 2); a polycarbonate resin composition containing a copolymerization-polycarbonate resin derived from 9,9-bis(4-(2-hydroxyethoxy)phenyl)fluorene and a prescribed hydroxy compound and a polycarbonate resin derived from bisphenol A in an amount has been proposed (see Patent Document 3); and the applications thereof have been also proposed (see Patent Document 4). Although these polycarbonate resins have lower birefringence compared with the polycarbonate formed of bisphenol A, they suffer from low strength.
Therefore, low-birefringent polycarbonate resins excellent in strength are required.